The maximum rate of sustained energy intake (SusEI) may limit reproductive effort, thermoregulatory capability and other aspects of an animal's energy expenditure. Consequently, factors that limit SusEI are of interest. The ;heat dissipation limitation hypothesis' suggests that maximum SusEI during lactation is limited by the capacity to dissipate body heat generated as a by-product of processing food and producing milk. In the present study, we tested the heat dissipation limitation hypothesis in lactating Brandt's voles (Lasiopodomys brandtii). Female voles were mated and pregnant at 21(+/-1) degrees C. A random sample of animals was transferred into a hot room 30(+/-1) degrees C on the day of parturition. The energy intake of lactating voles at 30 degrees C was always lower than that at 21 degrees C. At peak lactation food intake was 3.3 g day(-1) lower at 30 degrees C than at 21 degrees C. There was no significant difference in digestibility. With similar mean litter sizes (7.26+/-0.46 pups at 21 degrees C and 7.78+/-0.39 pups at 30 degrees C at the beginning of parturition, 6.83+/-0.51 pups at 21 degrees C and 7.73+/-0.50 pups at 30 degrees C at weaning), the milk energy output of mothers, evaluated from the difference between metabolizable energy intake and daily energy expenditure measured by doubly labelled water, at 30 degrees C was 23.3 kJ day(-1) lower than that at 21 degrees C on days 14-16 of lactation. As for reproductive performance, there was a difference in the response to the higher temperature between mothers raising large and those raising small litters. For small litters (<7) there was no significant change in litter mass, but for large litters (<or=7) there was a significant decrease at the higher temperature. On average, in larger litters the pups were 15.5 g heavier on day 12 of lactation when raised at 21 degrees C. Our data from Brandt's voles support the suggestion that SusEI at peak lactation is limited by heat dissipation capacity, particularly for those voles raising large litters. In smaller litters the peripheral limitation hypothesis may be more relevant. The importance of heat dissipation limits in species raising exclusively small litters needs to be investigated.